1. Field of the Invention
The present invention relates to a ceramic substrate, and more particularly, to a ceramic substrate for mounting thick-film hybrid ICs or high speed LSI elements.
2. Description of Related Art
According to the recent tendency of reducing size and weight of electronic devices such as communication equipment, home-use electric and electronic device, computer, etc., and of increasing number of functions, high-density and high-reliability of them, wiring technique for mutually connecting semiconductor chips has become more important in addition to high integration and high speed operation of these chips. As a substrate material, it can be roughly classified to an organic resin group, metal group and ceramic group. Among them, the ceramic group is frequently used in fields in which high quality, high reliability and long duration are required. Among ceramic materials, alumina has been widely used in views of its electric characteristics, its chemical stability, its mechanical characteristics and its cost. However, in order to use the green sheet multilayer method by which three dimensional wiring is possible and which is advantageous in realizing high density, conductor material of wiring layer is limited to metal such as tungsten or molybdenum whose specific resistance is relatively high since it must be heated to temperature as high as sintering temperature of alumina of 1500.degree. to 1600.degree. C.
In view of this, a low sintering temperature ceramic which is a composite material of glass and host ceramic has been developed. Since this composite material has a low melting point, metal material as wiring layer whose specific resistance is low and thus it can not be sintered simultaneously with sintering of ceramic material becomes usable. As such metal material as wiring layer, gold, silver and copper, etc., may be considered. However, gold and copper do not occupy large part of hybrid IC substrate market in views of high cost for gold and unstability against oxidation for copper and thus it can be said that silver or silver containing material is mainly used as wiring material. However, the wiring layer made of silver or of material containing silver has a migration problem, and to cope this problem a silver alloy with palladium or like has been used.
Although proportion of silver to palladium has been selected usually as 80:20 in views of anti-migration, solder-wettability, solder loss, adhesiveness and resistance value, it has been desired to use conductive material containing higher silver ratio in order to lower the resistance value and improve the solder wettability and aging characteristics. Solder loss, bad matching with cross over glass, degradation of solder wettability due to surface galvanization and silver migration can be avoided in various manners and thus it has high possibility of future development.
On the other hand, a multi-layered glass ceramic substrate in which wiring length can be reduced and high density wiring is possible is payed attention in multichip package which is used in a large, high speed computer and in which a large number of LSI chips are mounted on a single substrate. As wiring material in such multi-layered glass ceramic substrate, the above-mentioned silver or material containing silver is usually used. However, in order to further increase the mounting density, there is a tendency that the size of substrate becomes large. In order to sinter such large substrate in the same way as that of the conventional one, it is necessary to make the sintering time longer. However, this may enhance diffusion of silver of wiring into glass substrate during sintering process, resulting in a problem of remarkable degradation of various characteristics of the wiring of silver or material containing silver.